1. Field of the Invention
The present invention relates to a liquid electrophotographic development sheet, and in particular to a liquid electrophotographic development sheet which has excellent properties of transferring and fixing a toner.
2. Prior Art
Conventionally, as a method of reproducing a large number of copies of information such as characters and pictures, a printing method such as offset or gravure has been well known. According to the conventional printing method, a plate on which information to be recorded has been reproduced is formed, and then information which was represented by the presence of ink or a variable density thereof is reproduced from the plate to a substrate sheet. Since a working process requires very much labor and time, the Conventional printing method is not suitable for reproducing various types of information little by little in various combinations, although a much superior method of reproducing a large number of copies of information.
Therefore, an electronic printing method which can be represented by a copying machine has come to the main current as a method of reproducing a small number of copies of information. The electronic printing method is suitable for reproducing a small number of copies of information which is a weak point of the conventional printing method, since there is no necessity of photoengraving. As the electronic printing method, various methods such as a thermal system, an electrophotographic system, a thermal-transferring system and an ink-jet system have been proposed, and a copying machine, a printer, a facsimile telegraph and the like to which such a system is applied are actually coming on to the market in large quantities.
At the present time, with respect to the reproduction of information of a characters and simple illustrations level, it has become very common to use a monochromatic copying machine, while with respect to the printing of an original picture having gradations such as graphics, or an original picture in full color, the conventional printing method is still often used. This is, because the electronic printing method is often inferior in the quality of pictures, cost, speed and the like.
However, recent developments in electronics are remarkable, and thereby, electronic printing technologies also are rapidly developed, and some printing machines have been proposed, which utilize electronic printing methods and can provide a rapid reproduction with pictures of higher quality at a low cost.
Among the electronic printing methods, a dry electrophotographic system is the one which can be represented by a business copying machine and the like, in which a toner for forming pictures is a powdered solid toner which comprises a pigment and a synthetic resin; and the formation of the pictures is carried out by making the toner adsorbed to an electrostatic latent image which was formed on the surface of a photosensitive body by a corona electrical charging, and heating and applying the toner to a development sheet.
This dry electrophotographic system is not good for the environment in the point that the more finely the toner is grained, the more easily the toner can be scattered, and the inhaling the toner is not good for the health. Therefore, a limit is imposed on finely graining the powdered solid toner, which is a obstacle to provide a high definition. In addition, the dry electrophotographic system has many problems. For example, when the thickness of an object to be transcribed is inhomogeneous, the charge density on the surface of the object to be transcribed, which was provided by a corona electrical charging, is dispersed, and consequently, an undesirable image which is generally referred to as "fog" is caused on a record sheet at a non-picture portion. In addition, it is necessary to apply a high temperature so as to fuse the toner.
In the ink-jet system, an image is formed by spattering fine ink-drops onto a development sheet so as to form an image. However, the system inherently involves a problem that, in order to form a color picture, it is necessary to use an aqueous ink which contains a dye staff having a poor light-resistance and water-resistance Moreover, it is necessary to use a special purpose recording-sheet having a high liquid-absorbing ability.
In the thermal system or a thermal transfer system, a special development sheet or an ink ribbon needs to be used, and therefore, it is important to get free of incurring a high cost.
Accordingly, in due consideration of satisfying conditions such as a high-quality picture, a high speed and a low cost, the liquid electrophotographic system is one of the most promising system among various electronic printing systems at present.
That is, in the liquid electrophotographic system, since the toner can be more fine as compared with that used in a dry electrophotographic system, and since a pigment can be used as a color material, there is no problem in light-resistance and water-resistance.
Next, a printing machine for the liquid electrophotographic system will be explained. The liquid electrophotographic system has been investigated from a fairly long time ago, and some machines have been put to practical use. However, such machines were not popularized, since problems in a solvent smell, the ability thereof to transfer to a base or substrate sheet and the like had arisen. However, recently, as a machine in which the various defects as mentioned above has been removed, for example, a digital printing machine such as the registered trademark "E-PRINT" made by INDIGO Corp. has been provided.
Such a digital printing machine will be explained with reference to FIG. 1. The digital printing machine, which is similar to a normal offset printing machine in the construction, has three cylinders, i.e., a drum 1, a blanket cylinder 2 and an impression cylinder 3. The drum 1 comprises a photoconductor, and the blanket cylinder 2 is constituted to be heated in order to form a fused film from a liquid toner.
First, the drum 1, which was charged by a corona electrical charging, is exposed by a light beam in a dark place so as to form a latent image. Since the exposure is carried out by a laser light source according to a bit-map information from a computer, no physical preparation is necessary to form a latent image. Therefore, as compared with the conventional offset printing, the advantageous effect is provided that it is possible to print a different information for every number of each sheets to be printed, and no complicated process for plate-making is necessary, and furthermore, no expense for plate-making is incurred.
A latent image having an electric charge formed on the drum 1 is supplied with a liquid toner which was made to carry the contrary electric charge from an injector 4 and adsorbs the toner to form an ink picture by the toner. Thereafter, the ink picture is supplied from the drum 1 to the blanket cylinder 2 by static electricity. The ink picture is heated to be fused so as to form a film on the blanket cylinder 2. Lastly, the impression cylinder 3 is used to transfer the film-shaped picture to an article to be transferred by pressure so as to form an objective picture. In a full color printing, the steps mentioned above are repeated four times, while each of liquid toners of yellow, cyan blue, magenta and black is used.
Thus, a digital printing machine to which the liquid electrophotographic system is applied is a new printing machine having an excellent advantageous effect. However, in order to provide the maximum of the characteristics thereof, a particular property is demanded for a development sheet to be used. Namely, there is a problem that when a printing sheet which has been used as a development sheet in the conventional printing method was used, the transferring property of a toner to the development sheet is not satisfactory, because the adhesion between the toner and the surface of the development sheet is weak. Moreover, even if the toner is transferred, the toner will be easily peeled by an external force such as a frictional force or a scratch.
Therefore, it is an object of the present invention to provide a liquid electrophotographic development sheet which is excellent in the transferring and fixing properties of a toner, and can be put to practical use.